A polymerizable compound having a liquid crystal phase provides a polymer having such a function as optical compensation through polymerization. This is because the alignment of the liquid crystal molecules is fixed by polymerization. In order to utilize the function of the polymer, various kinds of polymerizable compounds have been developed. However, sufficient functions may not be obtained with only one polymerizable compound. Accordingly, there have been attempts that a composition is prepared by using plural polymerizable compounds, and the composition is polymerized (as described in JP-A-10-319408 (GB 2324382A), JP-A-2004-198478, JP-A-2002-243942 and JP-A-2005-196221).
The situation that an alignment state of a liquid crystal skeleton is in homogeneous alignment, tilted alignment, homeotropic alignment or twisted alignment is sometimes referred simply to as such an expression that a polymer has homogeneous alignment, tilted alignment, homeotropic alignment or twisted alignment, respectively.
A polymer having homeotropic alignment has an optical axis in an nz direction, and the refractive index in the direction of the optical axis is larger than the refractive index in the direction perpendicular to the optical axis. Accordingly, the polymer is classified into a positive C plate in terms of optical indicatrix. The positive C plate can be applied, by combining with a film having the other optical function, to optical compensation of a liquid crystal mode with horizontal alignment, i.e., a so-called IPS (in-plane switching) mode, and the like, such as improvement in viewing angle characteristics of a polarizing plate (as described in M. S. Park, et al., IDW, '04, FMC8-4, M. Nakata, et al., SID, '06, P-58, WO 05/38517 and US 2006/182900).
In the aforementioned purposes, a polymerizable liquid crystal material may be laminated with a glass substrate or a plastic substrate in some cases. Examples of the material used for the plastic substrate include TAC (triacetyl cellulose), polycarbonate, PET and a cycloolefin polymer.
In order to make a polymerizable liquid crystal in homeotropic alignment, there are such methods, in the case where the supporting substrate is a glass substrate, as a method of selecting the structure of the polymerizable liquid crystal compound to exhibit a smectic phase (as described in JP-A-2000-514202 (U.S. Pat. No. 6,379,758 B1)), and a method of coating lecithin as a homeotropic alignment film on the glass substrate (as described in JP-A-7-294735 (U.S. Pat. No. 5,863,457)). In the case where the supporting substrate is a plastic substrate, it is necessary to form an alignment film on the supporting substrate (as described in JP-A-10-319408 (GB 2324382A) and WO 04/72699). The inventors have found such a polymerizable liquid crystal composition that a polymerizable liquid crystal compound can be formed on a glass substrate or a plastic substrate to have homeotropic alignment without formation of an alignment film on the supporting substrate and can be excellent in adhesion property (JP-A-2006-126757 (US 2005/224754 A1)). However, the composition may suffer increase of the viscosity of the solution to provide a problem in storage stability, and thus it is necessary to mix an additive immediately before use. Furthermore, there are some cases where uniform alignment property cannot be obtained depending on the kind of the supporting substrate. Accordingly, such a polymerizable liquid crystal composition has been demanded that provides homeotropic alignment on a glass substrate and a plastic substrate without a surface treatment with a homeotropic alignment film having long-chain alkyl groups or the like or an inorganic material, and particularly is excellent in storage stability of the solution.